Scala.js: Implement support for js.new.target.

Forward port of the upstream commit
scala-js/scala-js@d011393

Author: sjrd

compiler/src/dotty/tools/backend/sjs/JSCodeGen.scala

@@ -3791,6 +3791,19 @@ class JSCodeGen()(using genCtx: Context) {
         // BoxedUnit.UNIT, which is the boxed version of ()
         js.Undefined()
 
+      case JS_NEW_TARGET =>
+        // js.new.target
+        val valid = currentMethodSym.get.isClassConstructor && currentClassSym.isNonNativeJSClass
+        if (!valid) {
+          report.error(
+              "Illegal use of js.`new`.target.\n" +
+              "It can only be used in the constructor of a JS class, " +
+              "as a statement or in the rhs of a val or var.\n" +
+              "It cannot be used inside a lambda or by-name parameter, nor in any other location.",
+              tree.sourcePos)
+        }
+        js.JSNewTarget()
+
       case JS_IMPORT =>
         // js.import(arg)
         val arg = genArgs1

compiler/src/dotty/tools/backend/sjs/JSDefinitions.scala

@@ -145,6 +145,11 @@ final class JSDefinitions()(using Context) {
     @threadUnsafe lazy val JSConstructorTag_materializeR = JSConstructorTagModule.requiredMethodRef("materialize")
     def JSConstructorTag_materialize(using Context) = JSConstructorTag_materializeR.symbol
 
+  @threadUnsafe lazy val JSNewModuleRef = requiredModuleRef("scala.scalajs.js.new")
+  def JSNewModule(using Context) = JSNewModuleRef.symbol
+    @threadUnsafe lazy val JSNew_targetR = JSNewModule.requiredMethodRef("target")
+    def JSNew_target(using Context) = JSNew_targetR.symbol
+
   @threadUnsafe lazy val JSImportModuleRef = requiredModuleRef("scala.scalajs.js.import")
   def JSImportModule(using Context) = JSImportModuleRef.symbol
     @threadUnsafe lazy val JSImport_applyR = JSImportModule.requiredMethodRef(nme.apply)

compiler/src/dotty/tools/backend/sjs/JSPrimitives.scala

@@ -24,7 +24,9 @@ object JSPrimitives {
 
   inline val UNITVAL = JS_NATIVE + 1 // () value, which is undefined
 
-  inline val JS_IMPORT = UNITVAL + 1        // js.import.apply(specifier)
+  inline val JS_NEW_TARGET = UNITVAL + 1 // js.new.target
+
+  inline val JS_IMPORT = JS_NEW_TARGET + 1  // js.import.apply(specifier)
   inline val JS_IMPORT_META = JS_IMPORT + 1 // js.import.meta
 
   inline val CONSTRUCTOROF = JS_IMPORT_META + 1                         // runtime.constructorOf(clazz)
@@ -105,6 +107,8 @@ class JSPrimitives(ictx: Context) extends DottyPrimitives(ictx) {
 
     addPrimitive(defn.BoxedUnit_UNIT, UNITVAL)
 
+    addPrimitive(jsdefn.JSNew_target, JS_NEW_TARGET)
+
     addPrimitive(jsdefn.JSImport_apply, JS_IMPORT)
     addPrimitive(jsdefn.JSImport_meta, JS_IMPORT_META)
 

project/Build.scala

@@ -1218,6 +1218,7 @@ object Build {
             )).get
 
           ++ (dir / "js/src/test/require-2.12" ** "*.scala").get
+          ++ (dir / "js/src/test/require-new-target" ** "*.scala").get
           ++ (dir / "js/src/test/require-sam" ** "*.scala").get
           ++ (dir / "js/src/test/scala-new-collections" ** "*.scala").get
           ++ (dir / "js/src/test/require-no-modules" ** "*.scala").get

tests/neg-scalajs/js-new-target.scala

@@ -0,0 +1,41 @@
+import scala.scalajs.js
+import scala.scalajs.js.annotation.*
+
+object IllegalInScalaClass {
+  class A {
+    js.`new`.target // error: Illegal use of js.`new`.target.
+
+    def this(x: Int) = {
+      this()
+      js.`new`.target // error: Illegal use of js.`new`.target.
+    }
+  }
+
+  class B {
+    def foo(x: Int): Unit =
+      js.`new`.target // error: Illegal use of js.`new`.target.
+  }
+
+  class C extends js.Object {
+    class D {
+      js.`new`.target // error: Illegal use of js.`new`.target.
+    }
+  }
+}
+
+object IllegalInDefOrLazyVal {
+  class A extends js.Object {
+    lazy val x = js.`new`.target // error: Illegal use of js.`new`.target.
+    def y: js.Dynamic = js.`new`.target // error: Illegal use of js.`new`.target.
+    def z(x: Int): Any = js.`new`.target // error: Illegal use of js.`new`.target.
+  }
+}
+
+object IllegalInLambdaOrByName {
+  class A extends js.Object {
+    val x = () => js.`new`.target // error: Illegal use of js.`new`.target.
+    val y = Option(null).getOrElse(js.`new`.target) // error: Illegal use of js.`new`.target.
+    val z: js.Function1[Int, Any] = (x: Int) => js.`new`.target // error: Illegal use of js.`new`.target.
+    val w: js.ThisFunction0[Any, Any] = (x: Any) => js.`new`.target // error: Illegal use of js.`new`.target.
+  }
+}